Abstract
BackgroundProton stress and aluminum (Al) toxicity are major constraints limiting crop growth and yields on acid soils (pH < 5). In Arabidopsis, STOP1 is a master transcription factor that controls the expression of a set of well-characterized Al tolerance genes and unknown processes involved in low pH resistance. As a result, loss-of-function stop1 mutants are extremely sensitive to low pH and Al stresses.ResultsHere, we report on screens of an ethyl-methane sulphonate (EMS)-mutagenized stop1 population and isolation of nine strong stop1 suppressor mutants, i.e., the tolerant to proton stress (tps) mutants, with significantly enhanced root growth at low pH (4.3). Genetic analyses indicated these dominant and partial gain-of-function mutants are caused by mutations in single nuclear genes outside the STOP1 locus. Physiological characterization of the responses of these tps mutants to excess levels of Al and other metal ions further classified them into five groups. Three tps mutants also displayed enhanced resistance to Al stress, indicating that these tps mutations partially rescue the hypersensitive phenotypes of stop1 to both low pH stress and Al stress. The other six tps mutants showed enhanced resistance only to low pH stress but not to Al stress. We carried out further physiologic and mapping-by-sequencing analyses for two tps mutants with enhanced resistance to both low pH and Al stresses and identified the genomic regions and candidate loci in chromosomes 1 and 2 that harbor these two TPS genes.ConclusionWe have identified and characterized nine strong stop1 suppressor mutants. Candidate loci for two tps mutations that partially rescue the hypersensitive phenotypes of stop1 to low pH and Al stresses were identified by mapping-by-sequencing approaches. Further studies could provide insights into the structure and function of TPSs and the regulatory networks underlying the STOP1-mediated processes that lead to resistance to low pH and Al stresses in Arabidopsis.
Highlights
Proton stress and aluminum (Al) toxicity are major constraints limiting crop growth and yields on acid soils
A large portion of false putative tps mutants from the initial screens could be due to multiple factors, including environmental effects and high density of seedlings at the initial screen, which could jeopardize the accuracy of the initial identification of the stop1 suppressor mutants
We notice that in stop1, Al treatment caused small, but significant, increases in ALMT1 and multidrug and toxic compound extrusion (MATE) transcript levels, whereas ALS3 expression was not affected by Al treatment (Fig. 5b, c and d). These results suggest that Sensitive to proton rhizotoxicity 1 (STOP1) plays a key role in controlling the Alinduced expression of ALMT1 and MATE, there exist nonSTOP1 regulatory factors that control a smaller portion of Al-induced ALMT1 and MATE expression, whereas the expression of ALS3 is likely to be solely controlled by STOP1
Summary
Proton stress and aluminum (Al) toxicity are major constraints limiting crop growth and yields on acid soils (pH < 5). Acid soils are associated with excess levels of toxic ions such as aluminum (Al3+), manganese (Mn2+), and proton (H+), which cause stunted growth and significant yield reductions of crops grown on acid soils [1,2,3]. Through the internal Al tolerance mechanisms, Al retained in the root cell wall is actively removed by Al transporters, such as NRAT1 in rice [11, 12] and NIP1;2 in Arabidopsis [13], into the root cytosol. Al in the root cell cytosol is further sequestered into root cell vacuoles and/or translocated and stored in the
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